Air Quality

Increases in Airborne Allergen Exposure

The frequency and severity of allergic illnesses, including asthma and hay fever, are likely to increase as a result of a changing climate. Earlier spring arrival, warmer temperatures, changes in precipitation, and higher carbon dioxide concentrations can increase exposure to airborne pollen allergens.

Climate change, specifically rising temperatures and increased CO2 concentrations, can influence plant-based allergens, hay fever, and asthma in three ways: by increasing the duration of the pollen season, by increasing the amount of pollen produced by plants, and by altering the degree of allergic reactions to pollen.

Seasonally, airborne allergen (aeroallergen) exposure in the United States begins with the release of tree pollen in the spring. Between the 1950s and the early 2000s, warming winters and earlier arrival of springs have resulted in earlier flowering of oak trees.113 Projected increases in CO2 induce earlier and greater seasonal pollen production in pine trees114 and oak trees.115 For summer pollen producers, such as weeds and grasses, the effect of warming temperatures on earlier flowering is less evident. However, the allergen content of timothy grass pollen increases with concurrent increases in ozone and CO2.116 For common ragweed, the primary fall aeroallergen, greenhouse studies simulating increased temperature and CO2 concentrations resulted in earlier flowering, greater floral numbers, increased pollen production, and enhanced allergen content of the pollen.117,118,119,120 Regional and continental studies indicate that ragweed growth and pollen production increase with urban-induced increases in temperature and CO2. Ragweed pollen season exposure varies as a function of latitude and delayed autumnal frosts in North America.119,121 In addition to pollen, aeroallergens are also generated by molds. Plants are often affected, since they can serve as hosts for fungi. For example, projected end-of-century CO2 concentrations would substantially increase the number of allergenic spores produced from timothy grass.122

Although warming temperatures and rising CO2 levels clearly increase aeroallergen prevalence, the link between exposure and health impacts is less well established. However, hay fever prevalence has been associated with exposure to annual and seasonal extreme heat events.123 Furthermore, climate-induced changes in oak pollen are projected to increase the number of asthma-related emergency department visits in the Northeast, Southwest, and Midwest.115